1. Field of the Invention
The invention concerns a measuring transformer for comparing a current flowing through a conductor to a reference current comprising a magnetic circuit formed by a toroidal core, a conductor through which the current flows and which is enclosed by the toroidal core, a secondary winding arranged on the toroidal core, and a magnetic field measuring element which is arranged in a gap of the toroidal core and which is sensitive to the magnetic field in the gap. The invention further concerns a control unit for controlling or regulating a current flowing through a conductor, an inverter, in particular for a wind power installation, as well as a wind power installation having such an inverter.
2. Description of the Related Art
A measuring transformer of the kind set forth in the opening part of this specification is known from EP 0 194 225. In the case of that measuring transformer the amplified output signal of a Hall element which serves as a magnetic flux measuring element feeds the secondary winding. The winding direction thereof is so selected that the magnetic field produced is in opposite relationship to the magnetic field surrounding the conductor through which the current flows. In that case the secondary winding is fed by the amplifier in such a way that it tries to make zero the magnetic field produced by the conductor. The current through the secondary winding is used as a measurement in respect of the current flowing in the conductor, that is to say the output signal of the secondary winding gives the absolute value of the instantaneous current flow.
A further measuring transformer of the kind set forth in the opening part of this specification is known from Elektronik Industrie 8-2001, pages 49 and 51. In that measuring transformer also, a coil is again wound around the toroidal core, and the current flowing through the conductor again induces a current in the coil. That induced current is superimposed with a possible deviation detected by the Hall element, and thus again gives an absolute value as a measurement in respect of the current flowing in the conductor. It will be noted however that with this measuring transformer also, the main component of the induced current again flows through the secondary winding. The combination of the Hall element and the amplifier detects the respective proportion of the current which flows in the conductor and which the secondary winding on the toroidal core cannot detect.
A measuring transformer of the kind set forth in the opening part of this specification is also further known from EP 0 580 473 A1. EP 0 157 054 A discloses a fault current protection switch in which two turns are arranged on a toroidal core. A comparator is provided for comparing the output voltage of a Hall element arranged in an intermediate space of the toroidal core to a reference voltage. The switch is then controlled by means of the output voltage from the comparator.
The article by Ogasawara et al ‘A Digital Current Sensor for PWM inverters’, Proceedings of the Industry Applications Society Annual Meeting, Houston, Oct. 4, 1992, New York, US, Vol 1, 4th Oct. 1992, pages 949–955, in FIG. 2, discloses a further measuring transformer for analog currents, which approximately corresponds to the measuring transformer known from EP 0 194 225.
In the case of the known measuring transformers the respective output signal is subjected to further processing in any manner as it gives the absolute value of the instantaneous current flow through the conductor. By way of a downstream-connected comparator, that value can be compared for example to a reference or target value in order to derive therefrom control signals, for example for an inverter, the current of which is delivered by way of the above-mentioned conductor. As that current can readily amount to some hundreds of amperes—instantaneous peak values perfectly well reach around 750 amperes—, a suitably high number of ampere-turns must be reached with the secondary winding. In that case, the lower the current flowing through the secondary winding is intended to be, the correspondingly higher the number of turns must correspondingly be.
That however is also the serious disadvantage of the known structures. An inductance also always forms a time constant and thus limits the possible ways of rapidly following fluctuations in the current flowing in the conductor. A further aggravating circumstance is that the inductance of the secondary winding itself, by virtue of its typically inductive behaviour, makes rapid signal changes impossible.